There are various contexts in which powders must be distributed in a controlled manner in a given space. In the particular case of flue gas purification, a reactant in pulverulent form must frequently be introduced homogeneously within the flue gas to be purified. In general, the flue gas circulates in a duct or flue. The introduction of a powder may be carried out, in the simplest cases, by an injector located on the wall of the duct. The injector introduces the powder generally perpendicular to the flow direction of the flue gas, with a certain velocity. If the velocity of the powder is sufficiently high relative to the velocity of the flue gas, the powder will be able to be distributed sufficiently homogeneously in the flue gas. When the duct has large dimensions, typically having a diameter of several meters, it becomes difficult however to obtain a satisfactory distribution.
One possible solution to this problem is to use a multiplicity of injectors, penetrating at different depths inside the duct. This however increases the costs of the device.
It has then been envisaged to use a cylindrical pipe provided with orifices spread along the pipe, the pipe being placed in the middle of the flue gas, often perpendicular to its flow direction. The powder is introduced by pneumatic transport at one end of the pipe. It is distributed through outlet orifices, which makes it possible to distribute it more homogeneously within the flue gas. It has been observed that it is difficult to obtain similar flow rates through the various orifices, the orifices placed closest to the feed end of the pipe having a tendency to receive a higher flow rate. This defect is partly resolved by sealing the second end of the pipe, but then a gradual obstruction of the pipe by stagnant powder is witnessed. This unfavourable situation is particularly pronounced when the powder has a tendency to agglomerate, as is the case in the presence of moisture, for example (a powder classified as a class C powder by the Geldart test is said to be agglomerating). The particles constituting the powder are then also qualified as agglomerating. Furthermore, the use of a cylindrical pipe provided with conventional orifices has the disadvantage that the largest particles, in particular those having a diameter greater than about ten microns, make their way from the inside of the pipe to the orifices with more difficulty, which gives rise to a particle size segregation, the large particles having a tendency to stay in the pipe.
Described in U.S. Pat. No. 4,826,088 is a system for the pneumatic distribution, across the ground, of materials in powder form, used in agriculture. The system comprises cylindrical pipes, the cross section of which is split into two parts by welded plates positioned inside the pipe and extending to the outside of this pipe. The two corresponding flows are then directed separately by deflectors. The system does not enable distribution in more than two flows starting from a single pipe, which explains the need for several cylindrical pipes (3 in the variant represented). This system, which is difficult to introduce into a closed space such as a duct, does not enable a stable operation over a prolonged period, in particular when the powder used has a tendency to agglomerate, as it gradually gets blocked. Furthermore, it is complex and costly to produce, in particular when it is necessary to distribute the powder in more than two flows. The latter are then no longer correctly controlled and balanced.